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SKI-1601
Design of Unsteady Wind Tunnel
A Major Qualifying Project Report
Submitted to the Faculty of the
WORCESTER POLYTECHNIC INSTITUTE
in Partial Fulfillment of the Requirements for the
Degree of Bachelor of Science
in Aerospace Engineering
by
_____________________
Andrej Samardzic
_____________________
Jackson Gillenwaters
_____________________
Thomas Cieslewski
March 25, 2016
Approved by: _________________________________
Professor Seongkyun Im, Advisor
Aerospace Engineering Program
Mechanical Engineering Department, WPI
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Abstract
An unsteady wind tunnel was designed and built for the study of unsteady aerodynamics. The wind
tunnel was designed as an open-loop configuration, and an active flow control system was designed as a
series of horizontally aligned shutters placed in front of the test section which can block oncoming airflow
to create time-varying shear flows in the test section. The wind tunnel was constructed using primarily
acrylic and plywood and was run by an axial fan. The active flow control system was composed of four
shutters that were held in position and rotated using steel rods and actuators located just outside the
shutter frame. Computational Fluid Dynamics (CFD) was used throughout the design processes, ensuring
the generation of unsteady flows in the test section.
“Certain materials are included under the fair use exemption of the U.S. Copyright Law and
have been prepared according to the fair use guidelines and are restricted from further use."
3
Acknowledgements
This group would like to thank the following individuals for their help and support throughout
the entirety of this project.
Project Advisor Seongkyun Im
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Table of Authorship
Section Author
Abstract AS
Introduction JG,AS
Background
2.1 Wind Tunnels AS
2.2 Unsteady Aerodynamics AS
2.3 Active Flow Control Systems AS,TC,JG
Wind Tunnel Design AS
3.1 Manufacturing AS
Shutter System Design AS,TC,JG
4.1 Shutter System Testing JG
4.2 Shutter System Manufacturing JG,TC
CFD Analysis TC,AS
Conclusions and Recommendations
6.1 Conclusions AS
6.2 Recommendations AS
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Table of Contents
Abstract ......................................................................................................................................................... 2
Acknowledgements ....................................................................................................................................... 3
Table of Authorship ...................................................................................................................................... 4
Table of Contents .......................................................................................................................................... 5
List of Figures ............................................................................................................................................... 6
1. Introduction ........................................................................................................................................... 8
2. Background ........................................................................................................................................... 9
2.1 Wind Tunnels ................................................................................................................................ 9
2.2 Unsteady Aerodynamics ............................................................................................................. 12
2.3 Active Flow Control Systems ....................................................................................................... 13
3. Wind Tunnel Design ........................................................................................................................... 16
3.1 Wind Tunnel Manufacturing ....................................................................................................... 19
4. Shutter System Design ........................................................................................................................ 27
4.1 Shutter System Testing ............................................................................................................... 30
4.2 Shutter System Manufacturing ................................................................................................... 33
5. Computational Fluid Dynamics (CFD) Analysis ................................................................................ 37
6. Conclusions and Recommendations ................................................................................................... 41
6.1 Conclusions ................................................................................................................................. 41
6.2 Recommendations for Future Work ........................................................................................... 42
7. References ........................................................................................................................................... 44
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List of Figures
Figure 1: Outlining differences in wind tunnel design. Courtesy of NASA ................................................. 9
Figure 2: Color-coded model of an open-loop wind tunnel ........................................................................ 10
Figure 3: Cal-Tech fan-grid active flow control system ............................................................................. 14
Figure 4: Left-horizontally aligned shutters; Center-rhomboid-grid shutters; Right- vertically aligned
shutters ........................................................................................................................................................ 15
Figure 5: CAD model of Settling Chamber & Contraction Section ........................................................... 17
Figure 6: Wind tunnel design generated using CAD .................................................................................. 18
Figure 7: Flow velocity simulated using CFD ............................................................................................ 19
Figure 8: Completed wind tunnel settling chamber .................................................................................... 19
Figure 9: Second honeycomb structure, located at end of contraction section ........................................... 20
Figure 10: Completed contraction section, aerial view ............................................................................... 20
Figure 11: Completed contraction section, side view ................................................................................. 21
Figure 12: 1 completed side of diverging section-- Plywood sheet sanded and coated in primer and glaze
.................................................................................................................................................................... 21
Figure 13: Wooden corner guard connecting plywood sides of diverging section ..................................... 22
Figure 14: Completed wind tunnel test section ........................................................................................... 22
Figure 15: Construction of test section; connecting screws lined with Gorilla Glue visible on sheet 3 of
test section ................................................................................................................................................... 23
Figure 16: Hinge connection used in test section construction ................................................................... 23
Figure 17: Rubber gasket used to line test section connecting edges ......................................................... 24
Figure 18: Completed wind tunnel diffusing section .................................................................................. 24
Figure 19: Diffusing section extension, shown to fit fan enclosure ............................................................ 25
Figure 20: Spring attachment, not tensioned .............................................................................................. 26
Figure 21: Spring attachment, tensioned ..................................................................................................... 26
Figure 22: Fully assembled wind tunnel structure ...................................................................................... 26
Figure 23: Shutter section CAD model, side view ...................................................................................... 28
Figure 24: Shutter section CAD model, isometric view ............................................................................. 28
Figure 25: Shutter system acrylic frame ..................................................................................................... 29
Figure 26: Servo used in Shutter System Assembly ................................................................................... 29
Figure 27: Shutter Test Assembly Design .................................................................................................. 30
Figure 28 (Left): Fastening for Shutter ....................................................................................................... 32
Figure 29 (Right): Shutter Test Assembly Inside ....................................................................................... 32
Figure 30: Acrylic shutter system housing ................................................................................................. 33
Figure 31: Individual shutter, side view ...................................................................................................... 34
Figure 32: Close-up of screw connection in acrylic frame ......................................................................... 34
Figure 33: Actuators attached to outside of acrylic frame .......................................................................... 35
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Figure 34: Foam rollers used in shutter system assembly, side view ......................................................... 35
Figure 35: Contraction Section Design 1 (Left); Contraction Section Design 2 (Right) ............................ 37
Figure 36: Design 1 flow (Left); Design 2 flow (Right) ............................................................................. 38
Figure 37: Example of shear flow with original shutter design .................................................................. 39
Figure 38: CFD simulation, shutters open uniform flow similar to how most wind tunnels produce a flow
field ............................................................................................................................................................. 40
Figure 39: CFD simulation, top shutter closed ........................................................................................... 40
Figure 40: CFD simulation, top two shutters closed ................................................................................... 40
Figure 41: CFD simulation, top three shutters closed ................................................................................. 40
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1. Introduction
Unsteady aerodynamics involves time-varying flow conditions, and is regularly experienced in real-
world operations such as wind turbines, aircraft, and cars. For example, some of today’s wind turbines
have a height of 140 meters, and its blades have lengths of 60 meters alone. The velocity of wind gusts
differ significantly over ranges in height of this magnitude and can introduce fatigue loads on the turbine
blades, which can then lead to damage of its components over time. An understanding of unsteady
aerodynamic behavior is necessary in the calculation of products’ performances and loads, and the
innovation for the optimization for conditions they face in their environment [1].
The purpose of this project was to create an experimental facility for the study of unsteady
aerodynamics, and to conduct proof-of-concept testing for successful of time-varying flow conditions. To
narrow the scope of the project, and give a clear expectation of the conditions the testing would ideally
provide, the objective of the wind tunnel was to produce a sinusoidal shear flow in time through the wind
tunnel’s test section. The flow would provide researchers with added flexibility in experiments: a number
of standalone varying shear conditions, or a constantly changing shear flow. In order to successfully
achieve the goal, the project was divided into a number of elements: the wind tunnel, the active flow
control system, and the testing for proper unsteady aerodynamic conditions.
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2. Background
2.1 Wind Tunnels
A wind tunnel is an apparatus that allows researchers to move air over a body to simulate flight and
analyze aerodynamic properties of the flow such as lift and drag. Wind tunnels are designed to deliver a
consistent, steady stream of air to the test section and minimize turbulence.
Wind tunnels are essential to the study of aerodynamics. Since their founding in 1871 [2], they have
undergone a number of iterations in terms of design; the two most common designs are the open-loop
wind tunnel, and the closed-loop wind tunnel-- shown below in Figure 1. Both have the same essential
components, but their overall design and construction differ greatly. Open-loop wind tunnels draw air
from the ambient environment and exhaust it back to the ambient after exiting the fan, while closed-loop
wind tunnels create a circuit with air which repeatedly circulates through the tunnel. The closed-loop
wind tunnel design delivers improved efficiency and generates less noise, but is more expensive and more
difficult to manufacture.
Open-loop Design Closed-loop Design
Figure 1: Outlining differences in wind tunnel design. Courtesy of NASA
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For the project’s purposes, an open-loop wind tunnel design suffices performance criteria while adhering
to budget and manufacturing constraints and was therefore selected as the chosen design.
An open-loop wind tunnel consists of 5 primary components, listed from left to right on Figure 2
below: a settling chamber, contraction section, test section, diffusing section, and a fan [2].
Figure 2: Color-coded model of an open-loop wind tunnel
The first section of any open-loop wind tunnel is the settling chamber, designated in yellow above.
The purpose of the settling chamber is to reduce any turbulence in the air flow before entering the
converging section. In order to regulate turbulence, the section generally contains a honeycomb structure
and a series of woven-wire screens. Honeycombs are effective in reducing lateral turbulence, and
common designs include square, circular, or hexagonal-celled cross-sections. Hexagonal honeycombs are
most effective due to the smallest loss in pressure over the section. Following the honeycomb is one or
more woven screens, which are effective in reducing longitudinal turbulence.
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Non-turbulent air then enters the contraction section, denoted by the color green in Figure 2. The
purpose of the contraction section is to uniformly accelerate the air flow while minimizing turbulence. In
order to smoothly take high volume low-velocity air from the settling chamber and transfer it to a smaller
volume with a higher velocity, the shape of the section must be carefully chosen for the desired wind
tunnel performance.
Important values such as the contraction ratio, contraction semi-angles, and contour shape are
required to be calculated. The contraction ratio (N) is the ratio between the entrance and exit cross-
sectional areas of the contraction section, seen below in Equation 1.
𝑁 = (𝑊𝐶𝑖 × 𝐻𝐶𝑖) ÷ (𝑊𝑇 × 𝐻𝑇) ( 1 )
A larger contraction ratio leads to higher flow quality, but research indicated that a ratio of 4 is sufficient
in most industrial purposes [3]. The contraction semi-angles (α/2, β/2) are defined as the angles created
between the entrance and exit cross-sectional areas of the contraction section. These values can be
calculated using the length of the section, and the difference in height between cross-sectional areas, as
seen in Equations 2 and 3:
𝛼
2= tan−1((𝑆𝐿𝐶𝑖 − 𝐻𝑇) ÷ 𝐿𝐶)
( 2 )
𝛽
2= tan−1((𝑆𝐿𝐶𝑖 − 𝑊𝑇) ÷ 𝐿𝐶)
( 3 )
Contraction semi-angles of approximately 12 degrees provide reasonable flow quality, while
maintaining a reasonable contraction section length [3]. The contour shape describes the shape of the
contraction section as it moves from entrance to exit. The most effective contour shape holds the form of
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a third-degree polynomial, as shown in Figure 2, but in many low-cost wind tunnel designs, simple
straight-line ducts are used.
Following the contraction section comes the test section, shown in Figure 2 by the color blue. The
purpose of the test section is to allow researchers to analyze the airflow over the test object. The primary
requirement of the test section is to allow air to flow through it undisrupted. Beyond this requirement, the
design of the test section directly correlates to the application of the wind tunnel; factors taken into
consideration are test section size, length, and cross-sectional shape.
Air flows out of the test section into the diffusing section, indicated in Figure 2 by the color purple.
The diffusing section acts conversely to the contraction section—it takes low volume, high-velocity air,
and expands it uniformly across an increasing cross-sectional area to decelerate the flow while creating
minimal noise. In order to achieve this, it is important to consider the semi-opening angle (θ/2), which is
the angle created between the entrance and exit cross-sectional areas of the diffusing section, and can be
calculated using Equation 4 below:
𝜃
2= tan−1((𝑆𝐿𝐷𝑒 − 𝐻𝑇) ÷ 𝐿𝐷)
( 4 )
A semi-opening angle of less than 3.5 degrees provides the greatest efficiency, but research reveals that
an acceptable range for this value is between 5 and 10 degrees [3].
Finally, the fan, indicated in the figure by the color red, acts as a catalyst to the wind tunnel system by
pulling air through the test section at a desired operating speed.
2.2 Unsteady Aerodynamics
The study of unsteady aerodynamics is defined as the study of conditions in which the flow of air is
time dependent. The study of unsteady aerodynamics is of one of great value to the aeronautical industry
today; an understanding of unsteady flow field allows researchers to assess and improve a product’s
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performance in real-world conditions. Major examples of industries in which unsteady aerodynamics
have become of great importance are in the automotive, aviation, and renewable energy industries,
particularly with wind turbines.
The goal of the current unsteady aerodynamics study is to gain an understanding of how changes in
shear flow affect an object’s performance in both immediate and long-term timeframes. This includes the
effect on aerodynamic performance such as lift-to-drag of an airplane flying in unsteady conditions, and
the aforementioned example of a wind turbine under unsteady conditions and the introduction of fatigue
loads which can lead to component damage.
There have been very few established test facilities dedicated to the unsteady aerodynamics study and
analysis of the unsteady flow behaviors. The development of such facilities could lead to further growth
in the field of unsteady aerodynamics and optimization of products, leading to highly efficient operation
of aerodynamic objects in the future.
2.3 Active Flow Control Systems
An active flow control system is a mechanism that allows researchers to create unsteady aerodynamic
conditions in a wind tunnel. The function of the systems is to be able to take a steady, controlled flow of
air after it passes through the settling and contraction sections, and manipulate it to create time-varying
shear and velocity disturbances in the test section.
Active flow control systems are critical to the study of unsteady aerodynamic and aerodynamic
effects on objects. Research identified two prominent designs: a push system, and a pull system [4]. The
push system, also known as a fan-grid design, places a grid of small, low-power fans spanning the cross-
sectional area of the entrance to the test section. Each fan is individually computer-controlled to produce a
14
specific air speed, allowing researchers to control air speed at any point in the test section. Below is an
image of this design, produced by researchers at California Institute of Technology.
Figure 3: Cal-Tech fan-grid active flow control system
The pull system, also known as a shutter system, uses a series of shutters aligned across the test
section, spanning the section’s cross-sectional area, and is designed to resist oncoming flow to produce a
velocity gradient. Each shutter is computer-controlled to align perpendicularly with the flow to impose a
resistance to the incoming flow, parallel to the air flow to allow the air to pass freely through the section,
or at any angle between to customize conditions produced in the test section. Examples of two different
shutter system designs are shown in Figure 5.
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Figure 4: Left-horizontally aligned shutters; Center-rhomboid-grid shutters; Right- vertically aligned shutters
Shutters can be designed in a number of approaches, each providing different levels of control over
the air flow. One method is to have shutters be rectangular in shape and be aligned across the cross-
section. Rectangular, parallel shutters allow for the production of shear flow in the vertical or horizontal
direction, contingent on orientation of placement. Another method consists of a grid of rhomboid-shaped
shutters spanning the cross-sectional area of the entrance to the test section. Rhomboid-shaped shutters
are capable of producing shear flow in the vertical, horizontal, and diagonal directions, providing
researchers with maximum theoretical flow control among different shutter designs.
In an early analysis of potential active flow control systems to be implemented, it was determined that
the wind tunnel will house a horizontally aligned shutter system design for the production of unsteady
conditions. This system meets performance criteria while adhering to budget and manufacturing
constraints.
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3. Wind Tunnel Design
The wind tunnel is composed of five primary sections: the settling chamber, converging section, test
section, diffusing section, and the fan. Several considerations had to be made in order to achieve desirable
wind tunnel properties. Design criteria for the wind tunnel are listed below:
Low cost
Portability
Ease of manufacturing
Test section dimensions (L𝑇xW𝑇xH𝑇): 24 x 12 x 12 inches
Maximum operating speed in the test section of 5 m/s
The following sections outline the design of each component of the wind tunnel.
The settling chamber cross-sectional area matches the dimensions of the converging section inlet, and
contains a hexagonal honeycomb and a woven-wire screen in order to reduce flow turbulence. A
hexagonal honeycomb was used for its efficiency over other honeycomb structures.
Research indicated that sufficient flow quality can be generated using a contraction ratio of N = 4
[2], contraction semi-angles of α/2 = β/2 = 12 degrees, and a straight line duct from entrance to exit of
the section. A CAD model of the settling chamber and contraction section is shown in Figure 3.
The dimensions of the inlet were calculated using Equation 5 below:
𝑊𝐶𝑖 × 𝐻𝐶𝑖 = 𝑆𝐿𝐶𝑖2 = 𝑊𝑇 × 𝐻𝑇 × 𝑁
( 5 )
𝑆𝐿𝐶𝑖2 = 12 × 12 × 4
𝑆𝐿𝐶𝑖 = 24 inches
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The length of the contraction section was calculated using Equation 6:
𝐿𝐶 = (𝑆𝐿𝑖 − 𝐻𝑇) ÷ tan 𝛼/2 ( 6 )
𝐿𝐶 = ((24 − 12) ÷ 2) ÷ tan 12
𝐿𝐶 = 26 inches
Figure 5: CAD model of Settling Chamber & Contraction Section
Provided the design criteria, the test section will be 24 inches in length, with a square cross-sectional
area of 144 in2. This section will be produced out of ¼ inch Plexiglas for ease of observing while running
an experiment.
Research indicates that an acceptable range for the semi-opening angle is between 5 and 10 degrees
[3]. Given the scope of the current project, a semi-opening angle (θ
2) = 6 degrees was used for acceptable
flow quality.
The exit dimensions of the diffusing section was calculated using the following equation:
𝑆𝐿𝐷𝑒2 = tan 𝜃/2 × 𝐿𝐷 + 𝐻𝑇 ( 7 )
𝑆𝐿𝐷𝑒2 = tan(6) × 24 + 12
𝑆𝐿𝐷𝑒 = 18 inches
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The final component in designing the wind tunnel system was to select a fan to move air through the
system. An axial fan was used for the project due to convention in the field of aerodynamics. Axial fans
pull air through the tunnel as opposed to pushing it, creating a smoother flow through the system. The
wind tunnel will be used exclusively in the study of low-speed aerodynamics, meaning the fan would
operate at a maximum speed of 5 m/s. In order to ensure a speed of 5 m/s in the test section of the wind
tunnel, the maximum fan flow rate was an important specification which was analyzed in choosing a fan;
the desired flow rate for the purpose of this project given the cross-sectional area of the test section and
the desired flow velocity was 0.45 cubic meters per second.
Compiling the chosen designs for each component described above, a CAD model and a CFD
simulation were generated for the unsteady wind tunnel system. The CAD model and corresponding CFD
results are shown in Figures 4 and 5, respectively.
Figure 6: Wind tunnel design generated using CAD
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Figure 7: Flow velocity simulated using CFD
Results obtained from the wind tunnel system modelling were promising, with minimal flow
separation through the system, and a successful 5 m/s flow speed, indicated by the light green color,
through the test section. Positive results in CFD modelling allowed the wind tunnel system design to then
be manufactured, keeping in mind the original design criteria of low-cost, portability, and ease of
manufacturing.
3.1 Wind Tunnel Manufacturing
Given the scope of the project and allotted budget, the wind tunnel body was manufactured primarily
out of plywood and acrylic. The wind tunnel was constructed component-by-component, with sections
attached together to complete its construction; a guide for the manufacturing of each section is below:
Settling Chamber
Figure 8: Completed wind tunnel settling chamber
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The settling chamber, shown above in Figure 5, was designed as a frame to hold the honeycomb
section, which is vital to flow straightening in a wind tunnel. During the manufacturing process, it was
determined that the cell size of each honeycomb cell was too large to effectively straighten the flow for
the purposes of the project. In order to remedy the issue, a second honeycomb section, with a much
smaller cell size, was inserted at the end of the contraction section. The second honeycomb section can be
seen below in Figure 6.
Figure 9: Second honeycomb structure, located at end of contraction section
Contraction Section
Figure 10: Completed contraction section, aerial view
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Figure 11: Completed contraction section, side view
First, the 1/8 inch plywood sheet was cut into 4 trapezoidal shapes with base lengths of 12 inches and
24 inches with a height of 27 inches, based on prior calculations. Individual pieces were then attached
together using wood glue and wooden corner guards. To further increase flow quality, the corners on the
interior of the contraction section were lined with duct tape.
Each individual piece was then sanded and coated in a primer and glaze in order to create a smooth
surface allowing for higher flow quality.
Figure 12: 1 completed side of diverging section-- Plywood sheet sanded and coated in primer and glaze
22
With all sides of the section cut and coated, wood glue was used to attach wooden corner guards to
the edges of each side of the section, and then to connect the individual sides together.
Figure 13: Wooden corner guard connecting plywood sides of diverging section
Test Section
Figure 14: Completed wind tunnel test section
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The test section was composed of four half inch thick acrylic sheets measuring 12 inches x 24 inches.
The sheets were connected along the longer edges using screws and glue for reinforcement.
Figure 15: Construction of test section; connecting screws lined with Gorilla Glue visible on sheet 3 of test section
All standalone sides of the test section were connected perpendicularly to one another in this fashion.
In order to allow researchers access to the test section, the final sheet was connected to one edge using
hinges, allowing it to be swung open.
Figure 16: Hinge connection used in test section construction
24
To then keep the test section airtight, the other edge was lined with a rubber gasket strip and clamped
to the bottom edge of the test section.
Figure 17: Rubber gasket used to line test section connecting edges
Diffusing Section:
Figure 18: Completed wind tunnel diffusing section
Similarly to the contraction section, the 1/8 inch plywood sheet was composed of four trapezoidal
sheets of 1/8” thick plywood coated in primer and glaze. The sheets for the diffusing section had base
25
lengths of 12 inches and 18 inches and a height of 25 inches-- based on prior calculations for the diffusing
section. Individual pieces were then attached together using wood glue and wooden corner guards. To
further increase flow quality, the corners on the interior of the contraction section were lined with duct
tape.
A Cincinnati PF-12 axial propeller fan was then placed at the end of the diffusing section to pull air
through the section. The fan had a diameter of 18 inches, and was housed in a metal enclosure measuring
24 inches by 24 inches.
Originally, the team had planned to create a tight fit around the 18 inch fan—having each side of the
diffusing section be tangential to the edge of the fan—but were forced to adjust with the fan’s delivery.
The fan has a pre-installed enclosure with side lengths of 24 inches, and a ‘shelf’ which extrudes 9 inches
from the fan, holding the fan’s motor. To adjust, an extension to the diffusing section was built to expand
the outlet area of the section, as evident in photos above, and allow room for the motor to remain as it was
installed when delivered. Figure 16 shows how the extension fits the fan enclosure to allow comfortable
space for the diffusing section.
Figure 19: Diffusing section extension, shown to fit fan enclosure
26
Assembling Sections
In selecting a method by which to connect the different sections of a wind tunnel, it is important to
ensure an airtight seal from one section to the next for smooth, uninterrupted airflow. Given the scope of
the project, it was also important to focus on maintaining portability for the entire system of the wind
tunnel.
In order to comply with aforementioned criteria, hook-springs were used in connecting components
of the wind tunnel together. This method allows the system to be modular—increasing mobility—and if
tensioned properly, could create a tight seal between components. To ensure this seal, contact points
between primary sections were lined with strips of rubber.
Figure 20: Spring attachment, not tensioned
Figure 21: Spring attachment, tensioned
An image of the completed wind tunnel system structure is shown below in Figure 19.
Figure 22: Fully assembled wind tunnel structure
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4. Shutter System Design
The shutter system consists of three main sections: the shutters, actuators, and a frame that houses the
other two sections. Design criteria for the shutter system are as follows:
Strength to withstand operating air flow speed
Cross-sectional dimensions measuring 11.5” x 11.5”
Ease of manufacturing
Low cost
The shutter section contains a stack of four horizontal panels which can be independently rotated to
allow or block the flow of air in a specific quarter of the test section. The sizing of the shutters was
limited by the test section’s 11.5 inch by 12.5 inch cross-sectional area. Since the face of the shutter
section had to be flush with the face of the test section, the shutters had to be slightly less than 11.5 inches
long to account for the thickness of the shutter housing. The number of panels selected also was restricted
by the cross sectional area of the test section. The design calls for four panels as there are enough to
maintain active control over the airflow, while also being large enough to allow a rod to be inserted into
either end to facilitate the panel’s rotation. Each panel’s, or shutter’s, cross-section holds an elongated
diamond shape as seen in figures 23 and 24. The elongated diamond shape allows air to flow over the
panel undisrupted when parallel to the flow (i.e., open). The shaping was influenced by common airfoil
models. However, the shape is mirrored over its midsection to allow minimal interference on both the top
and bottom of the shutter since its goal is not to produce lift.
28
Figure 23: Shutter section CAD model, side view
Figure 24: Shutter section CAD model, isometric view
The shutters are encompassed by a frame that lines up with the test section as seen in figure 25,
allowing the test section to be completely blocked off if every panel is shut. On the outside of the frame is
a series of shelves that holds the actuators up.
29
Figure 25: Shutter system acrylic frame
The actuators, which are simply motors, are used to control the rotation of the shutters as air flows
through the test section. The actuator selection was based off of cost, ease of use, and rotation strength.
Given the above design criteria, electric servomotor actuators were selected as they are cheap and easy to
use while still maintaining acceptable rotation strength. The servo operates at 4.8-6V, the maximum
torque is rated at 0.59 N-m (6.0V), with a maximum angular rate of 375 degrees/second.
Figure 26: Servo used in Shutter System Assembly
30
The design of the shutter system offers researchers the opportunity to create shear flow and test
objects in unsteady conditions. The shutter system enables the air flow to be carefully controlled before
being imaged in the test section. The control offered allows for the testing of more conditions that one
would come cross in day to day situations, which is something that is not offered in traditional wind
tunnels.
4.1 Shutter System Testing
A model shutter system assembly, comprising of one shutter and servo, was constructed and tested
before final manufacturing of this section. The purpose of the shutter test assembly was to ensure stability
of the servos through drag forces created on the shutter assembly. The shutter test assembly design was
fairly simple, with a U-shaped frame holding a single shutter in place for testing, as seen in Figure 27.
Figure 27: Shutter Test Assembly Design
31
Figure 27 shows a downwind cross section of the shutter test assembly inside the test section of a
wind tunnel. The grey rectangles correspond to the aluminum bars that make up the U-shaped frame,
which consists of two upright 12” x 1” x 1” bars, and one 14” x 1.25” x 1.25” bar as the base. A shutter is
placed in-between the two uprights, shown as the color blue. The shutter was attached to the sidebars by
two pieces of threaded wire. Holes were drilled in the sides of the shutter as well as the bars, allowing the
shutter to rotate freely. One piece of the threaded wire was attached to a Servo resting just outside the
wind tunnel. The original design for the shutter test assembly was going to be taped to the wind tunnel in
order to hold it in place while there was air flow. However, due to the build of the assembly, there was the
potential for the entire frame to rock back and forth in the wind because of its profile and low weight. In
order to increase stability, a plate was attached to the bottom of the assembly.
There are a few variations between the shutter test assembly and the final assembly. There is only one
shutter-servo pair for the test instead of the full four. The test section for the final assembly is also only
12” x 12”, while the test tunnel is four times as large in terms of cross-sectional area. This difference in
area throughout the testing process will not cause an inconsistency in airflow quality as excess air will
simply pass over the test assembly. Having only one shutter will produce a different air flow through the
test assembly compared to the final assembly, as the air won’t be interacting with the other shutters.
While there is a discrepancy, the purpose of the test was to ensure stability in the shutter through the force
of flowing wind. Therefore, using one shutter in a larger test section was not an issue with this test.
Test Results
The testing run on the shutter assembly involved running the wind tunnel at different speeds while
using the servo to rotate the shutter at different speeds as well. The shutter test assembly was secured in
32
the tunnel using a second wooden plate outside the wind tunnel and bolting the two plates together, as
seen in Figure 28 below.
Figure 28 (Left): Fastening for Shutter
Test Assembly
Figure 29 (Right): Shutter Test Assembly Inside
Wind Tunnel
The tunnel was run at speeds from 4 m/s up to 7.5 m/s (5 Hz to 9 Hz). Even though the operating
speed of the final wind tunnel is to be 5 m/s, the test was run at higher speeds because as the shutters in
the final assembly close, the area the wind can flow through is reduced, speeding up the wind. The servo
operates at 4.8-6V, the maximum torque is rated at 0.59 N-m (6.0V), with a maximum angular rate of 375
degrees/second. This allowed the testing of the precision (lower speed) and power (higher speed) of the
servo. By going through the range of servo speeds at each air flow speed, almost every possible
combination of scenarios that would occur in the final wind tunnel was tested.
Throughout the test, there was no visible vibration at all. Further testing was performed using a
vibration sensor, as seen in Figure 29, which was attached to the shutter to obtain measurable data.
However, the vibrations that the flow created on the shutter were so weak that the sensor was unable to
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pick up any meaningful data. The lack of significant data combined with the “eye test” led to the
conclusion that the servo and shutter combination was fully functional and that the servo could handle
vibrations if any did occur during the final experiment. With this, construction of the final shutter system
could begin.
4.2 Shutter System Manufacturing
Given the scope of the project and allotted budget, the shutter system was manufactured using acrylic
and foam. The shutter system was designed in three steps: first the shutter housing was built, then the
actuators were attached, and lastly the foam rollers were added. The manufacturing of each step is
outlined below.
Shutter Housing
Figure 30: Acrylic shutter system housing
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First, the 1/4'” thick acrylic as cut into four rectangles that were 12” x 3” (sides) and 11.5” x 3” (top
and bottom). Then, two of the cut pieces of acrylic had holes drilled into them in order to hold the
shutters. Each 11.5” x 2.66” shutter had holes drilled into both ends, as seen in Figure 31. The shutters
were then placed in between the acrylic with the holes aligned. A plastic rod was glued through the holes
in the acrylic and shutters to keep the shutters in place.
Figure 31: Individual shutter, side view
The remaining two pieces of acrylic were placed to form an 11.5” x 11.5” square around the four
shutters. All sides of the shutter housing were screwed together to hold the frame, as seen in Figure 32.
Figure 32: Close-up of screw connection in acrylic frame
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Actuators
Figure 33: Actuators attached to outside of acrylic frame
For each shutter, an actuator was placed on the outside of the housing as seen in Figure 33. The
actuators were connected by the 4-40 steel rods. While one end of the rod was glued in the shutters inside
the housing, the other end was inserted into the rotating piece of the actuator. Each actuator was held up
by two pins inserted into the outside of the shutter housing, also seen in Figure 33. The actuators are
controlled by a Pololu six channel micro controller connected over USB and powered by a DC power
source.
Foam Rollers
Figure 34: Foam rollers used in shutter system assembly, side view
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Two 11.5 inch x 3 inch pieces of foam were cut out of a larger chunk of foam. Then, each piece was
sanded down as shown in Figure 34 to fit a .46 inch section at the top and bottom of the shutter housing.
The foam rollers were required to fit these small sections at the top and bottom of the housing due to
small errors in the 3D printing of the shutters. The software used in printing was unable to produce the
thin ends of the shutters, causing each shutter to be slightly smaller than the original CAD model. This
left a gap that had to be filled when all the shutters were in a closed position. The rollers are designed to
block this gap, as well as be aerodynamic when the shutters are open and air is flowing over the rollers.
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5. Computational Fluid Dynamics (CFD) Analysis
Computational fluid dynamics (CFD) simulation software is a valuable tool which allows for the
simulation and analysis of fluid flow. It provides users with the opportunity to test flow quality in
simulated 3-D designs in order to determine which design would lead to best results.
CFD simulation through SolidWorks was used in designing the contraction section of the wind
tunnel. SolidWorks uses the k-ε turbulence model in order to calculate flow conditions. [5] Proper design
of the contraction section of a wind tunnel is essential to its construction and its effectiveness as an
experimental facility. 2-D and 3-D simulations were ran on the model initially, but based on the symmetry
of the model and computational time, a 2-D simulation was used with no effect on overall flow
calculations. Background research showed that many low-cost wind tunnels were constructed using a flat
contraction section, designated below in Figure 35 as Design 2, rather than one with a curve leading into
the test section, shown on the left in Figure 35 which is commonly seen in large-scale wind tunnels.
These two conditions were both analyzed in a CFD simulation, with results revealing that a flat-edge
contraction section, with contraction semi-angles of less than 12 degrees, as found in research, led to
acceptable flow quality for the scope of the current project.
Figure 35: Contraction Section Design 1 (Left); Contraction Section Design 2 (Right)
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CFD analysis showed that Design 1 causes a non-uniform flow profile, with air travelling faster in the
center of the test section and slower at the edges. This is an unwanted non-uniform flow, as compared to
unsteady requirements of the project, air flow would have to be uniform going into the active flow control
system, and then manipulated by the system to create a controlled shear flow in the test section.
Alternatively, Design 2 allows for a uniform flow to enter the test section, allowing for accurate flow
manipulation and proper experimentation.
Figure 36: Design 1 flow (Left); Design 2 flow (Right)
Computational design was also used in the design of the shutter system. The shutter system is an
11.5” x 11.5” x 4” structure that is placed at the inlet of the test section, this device uses four horizontal
shutters 11.5” x 2.875” with an oblong cross section that will minimal flow separation. These shutters can
be turned 180 degrees and are controlled with servos. This allows the user to set up a test procedure for
the servos to follow.
The original design had 4 shutters each 11.5” x 2.88”, enough to cover the entire shutter cross
sectional area, with overlap between each shutter to allow for no air flow. Under these conditions the
CFD results showed that the shutters were able to create sinusoidal shear flow in time. The shutters are
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extremly thin diamon shapes, to limit their impact on air flow. And the edges of the shutter section
created no boundary layer issues connecting to the test section.
Figure 37: Example of shear flow with original shutter design
There were issues with the manufacturing process of the shutters. The shutters were supposed to have
a width of 2.875” but came out to be 2.66” which caused a large gap between the shutters. The shutter
assembly required a modification to still be functional. If the shutters cannot create a ”sealed” wall flow
will slip past and affect the test section results. The issue was solved by placing the shutters slightly closer
together to still maintain the flow blockage which is necessary. If an elliptical shape, cut in half is placed
at the bottom and top of the shutter system this allows flow to pass through with minimal impact to the
flow itself. The only issue noticed with testing is there approximately (2/3) through the test section the air
seems to push back up, causing the opposite shear force. There is a “Prime spot” for testing within the
first 2/3 of the test section. Although based on the CFD simulations you can move the testing object back
and forth in the tunnel based on test requirements.
Figures 38-41 below depict CFD calculations based on our model, with possible testing
configurations, velocity readings in the stream wise direction range from 0-6 m/s. Throughout the CFD
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testing, using a rectangular computational domain allowed calculation time to decrease from roughly ten
minutes to two minutes, there were no differences in test section flow.
Figure 38: CFD simulation, shutters open uniform flow similar to how most wind tunnels produce a flow field
Figure 39: CFD simulation, top shutter closed
Figure 40: CFD simulation, top two shutters closed
Figure 41: CFD simulation, top three shutters closed
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6. Conclusions and Recommendations
6.1 Conclusions
The goal of this project was to create an experimental facility for the study of unsteady aerodynamics,
and to conduct proof-of-concept testing for successful recreation of these conditions. The team compiled
research on wind tunnel design, the study of unsteady aerodynamics, and the concept of active flow
control systems in order to understand the scope of the project and to prepare a building plan. Completed
research and Computational Fluid Dynamics (CFD) analysis were then used in the actual construction of
the unsteady wind tunnel system.
The wind tunnel structure was an open-loop design and was constructed primarily out of 1/8”
plywood for the settling, contraction and diffusing sections, and ½” acrylic sheets for the test section.
Wind tunnel components were attached in a modular fashion using springs and spring-hooks as
connectors, and the entire wind tunnel system sat on a table for maneuverability purposes.
The shutter system was a series of 4 horizontally aligned shutters aligned perpendicularly to the air
flow in the wind tunnel. Housing the shutter system was an acrylic frame measuring 11.5” x 11.5” with
two foam rollers located at the top and bottom of the section to fill gaps caused by a manufacturing error
and ensure flow quality. Actuators powered the rotation of the shutters through 4-40 steel rods and were
then programmed to create specific unsteady conditions for experimentation.
Unfortunately, given the allotted time dedicated to the project, the team was not able to go through
the testing phase to verify successful recreation of unsteady conditions in the wind tunnel test section.
Alternatively, a flow simulation was completed using Computational Fluid Dynamics (CFD) software
whose results looked promising.
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6.2 Recommendations for Future Work
While working through this unsteady wind tunnel Major Qualifying Project (MQP), many problems
that were not originally anticipated held up the progress of the project. The following section outlines
portions of the project that were not completed as well as recommendations for future work on the project
and ways to improve what was accomplished.
Omitted from this project—as mentioned previously in the conclusion—is the testing phase to verify
recreation of unsteady aerodynamic conditions in the wind tunnel’s test section.
Identified areas for future improvement center around the build quality of the wind tunnel. Our first
recommendation is to look into the use of higher quality materials in the construction of the wind tunnel
sections—notably the settling, contraction and diffusing sections. While our structure suffices for the test
conditions of low-speed aerodynamics, we are unsure of how effective our system would be under more
intense conditions. Better material would lead to more stability, and could have a smoother profile,
leading to better flow quality in the wind tunnel.
Our next recommendation is to purchase a new table or stand for the wind tunnel structure. An
important criteria for the system was maneuverability, but due to unforeseen problems and adjustments
made throughout the design and manufacturing processes, this was not accomplished. Our completed
wind tunnel measured nearly 30 inches longer than the table we intended to use; to solve this problem, we
built an extension of the table to support the overhanging section of the wind tunnel. This extension,
while necessary, hinders maneuverability of the system. To remedy this, we recommend the purchase of a
longer table, ideally with wheels on the legs so that the system can be easily moved and showcased.
Our final recommendation is to recruit or seek guidance from a peer who has a good understanding of
Electrical and Computer Engineering (ECE) for the completion of the wiring and testing. As Aerospace
Engineering majors, we found this to be complicated and in retrospect, dedicated a lot of our time
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attempting to gain an understanding of ECE over building our wind tunnel structure. We feel that adding
a team member who has an understanding of these fundamental concepts will make the team more
efficient, and will allow for more focus on the project and its successful completion.
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7. References
[1] King, Rudibert. Active Flow Control II: Papers Contributed to the Conference "Active Flow Control
II 2010", Berlin, Germany, May 26-28, 2010. Berlin: Springer, 2010. Print.
[2] "Wind Tunnel Parts." Wind Tunnel Parts. NASA, n.d. Web. Oct. 2015.
[3] Hernandez, Miguel A. Gonzalez, Ana I. Moreno Lopez, Artur A., Jose M. Perales Perales, Yuliang
Wu, and Sun Xiaoxiao. "Design Methodology for a Quick and Low-Cost Wind Tunnel." Wind Tunnel
Designs and Their Diverse Engineering Applications (2013): n. pag. Web. Nov. 2015.
[4] Gad-el-Hak, Mohamed. Flow Control Passive, Active, and Reactive Flow Management. Cambridge:
Cambridge UP, 2000. Print.
[5] Williams, David, Wesley Kerstens, Seth Buntain, Vien Quach, Jens Pfeiffer, Rudibert King, and
Gilead Tadmor. "Closed-Loop Control of a Wing in an Unsteady Flow." Closed-Loop Control of
Blood Glucose Lecture Notes in Control and Information Sciences (n.d.): 109-26. Web. Oct. 2015.
